We've seen that arthropods all have bilateral symmetry and segmented bodies. Another character inherited by all arthropods is a hard exoskeleton.

Arthropod Armor
The bodies of arthropods are supported, not by internal bones, but by a hardened exoskeleton made of chitin, a substance produced by many non-arthropods as well. In arthropods, the nonliving exoskeleton is like a form-fitting suit of armor. It is produced by the "skin" and then hardens into a protective outer-covering.

This exoskeleton is handy in some ways (it provides protection and prevents water loss), but is limiting in others. In order to grow, all arthropods must shed the exoskeleton and produce a new, larger one.